I've got a gambrel roof that is due for a re-roofing. The last time it was done about 21 years ago, we used dimensional shingles (Timberline) and the roofer said that the shingles that make the bend from the upper pitch to the lower pitch would lay down and seal - which they never completely did on the north side of the house. From what I understand, that's the way to do it with 3-tab shingles which are thinner and bend more easily.

So now we are looking for the right way to go (cuz we still don't want 3-tabs).
The search led me to drip edge...and that led me to vented drip edge.

One roofer strongly advised us not to go with the VDE (vented drip edge) ( I'm getting tired of typing it all out). He says water will find it's way in, especially in the winter. Another is willing to do it, but now I have doubts.

So, does this stuff do the job well?

And here's the second part of the question. How do I know if I REALLY need VDE?

I have a ridge vent at the top of my roof and the attic is dry, so I know the upper portion is vented ok, but I have no way of knowing if the lower portions (mansards?) are vented. They have soffits at the bottom, but I can't tell if they have airflow where they meet the upper roof

I would at the time of re-roofing, dig in from above and see if you have good spacing for airflow. If not you could add 2x4s on flat and re-sheet the the lower part of the roof giving you air flow from the bottom to the higher part. As your roof lasted 21 years I suspect the venting would be fine. I'm sure your roofer could come up with some flashing for the transition.

Thanks for responding. I just about gave up on this thread. Yeah, so based on the fact that the first roof lasted this long, we decided that no extra venting was needed. We don't really want to change the profile of the roof. It's just that this house used to be a ranch, and now it's a colonial. The contractor that did the add-a-level (for the previous owner) wasn't so good. We've found many problems, and that's why I questioned the roof situation. The roofer that we're hiring is going to turn the corner between upper & lower with flashing and then shingle up to the edge - shouldn't be too much flashing showing (we hope). We thought about a fascia board at the seam, but decided against it.

Still don't know the verdict on vented drip edge, though!

Oh yeah, we're going to put in rafter vents (propper vents) where the insulation ends at the roof line. Roofer says it can only help.

Vented drip edge is good if you don't have soffet vents. I would still have the roof remove the sheeting in one spot so you can see what you have. Easy repair for peice of mind and have plan (B) and (C) if you don't like what you find there.

I can't speak for all vented drip edge products, but I have installed the Air Vent Pro Flow product on one residential roof replacement project and the results are inline with my expectations (aka air flows as expected). This Texas one story house had a 3/12 pitch roof with two intersecting ridges (both of which had ridge vents installed) and we installed the vented drip edge only on the eaves that were parallel with the ridge vents. When we were done, the airflow was very noticeable at both the intake and exhaust vents, and the homeowner still brags to her friends a year later about how her air conditioning unit began cycling (rather than run continuously all day) during the summer after we installed the new ventilation.

The product we used was extremely sturdy, made from aluminum but much thicker than typical DIY flashings / vents, and it extended about six inches up the decking from the edge.

Regarding your concerns about vented drip edge allowing water infiltration, it's important to bend the product to match the slope of the roof so that the vent openings themselves are parallel to the ground (aka horizontal). Once that's done, there's very little (if any) additional risk of water entering the attic than with any other under-soffit vent that may be available.